Generally, lactic acid is an important organic acid with a wide range of applications including food additives such as food preservative, fragrance, or acidifier etc., and has been used broadly for industrial purposes such as cosmetics, chemistry, metals, electronics, fabrics, dyeing textiles, and pharmaceutical industries, etc. In addition, lactic acid is an essential ingredient of polylactic acid, one of biodegradable plastics, and thus the demand for lactic acid has been increasing significantly. It is also used as an important material for the productions of many chemical compounds including polylactic acid, acetaldehyde, polypropylene glycol, acrylic acid, 2,3-pentathione, etc. Specifically, D-type lactic acid is an essential ingredient for producing streocomplex PLA, which is an optical isomer required for the production of highly heat-resistant PLA.
Specifically, the method for producing lactic acid includes a traditional chemical synthesis and a biological fermentation process. When lactic acid is produced via the chemical synthesis, lactic acid is produced in the form of a racemic mixture consisting of 50% D-type lactic acid and 50% L-type lactic acid, and it is difficult to control the composition ratio, and thus polylactic acid produced therefrom may become an amorphous polymer having a low melting point, thereby imposing limitations on the development of their use. On the other hand, the biological fermentation process allows to selectively produce D-type lactic acid or L-type lactic acid depending on the strain used. Thus, the latter is preferred commercially because it is possible to produce a particular isoform of lactic acid.
Meanwhile, attempts have been made in order to improve the productivity of lactic acid via various gene manipulations using a saccharomyces sp. microorganism having D-lactic acid-producing ability, by introducing a gene of an enzyme for conversion into D-type lactic acid. Specifically, attempts have been made to improve the productivity of lactic acid by strengthening the activity of lactic acid dehydrogenase (LDH) while decreasing the activities of pyruvate decarboxylase (PDC), aldehyde dehydrogenase (ALD), and/or acetyl-CoA synthetase (ACS), and (U.S. patent application Publication Nos. 2012-021421, 2010-0248233, and 2006-0148050). However, the overall fermentation productivity was low due to the low cell growth of the lactic acid-producing strain.